travis-ci: Don't update homebrew.

Jobs on travis-ci.org are now failing because updating homebrew
packages on the xcode11 image takes longer than the allotted
job time. We could bump the image version, but I wanted to
try this fix first.
2 jobs for travis in 1 minute and 44 seconds
Status Name Job ID Coverage
  Build
passed autoconf #10951
docker

00:01:44

passed cmake #10952
docker

00:00:36